Differential Diagnosis and the Indomethacin Test

When you have severe, one-sided head pain, getting the right diagnosis can be difficult because several conditions look very similar. Doctors group these together as Trigeminal Autonomic Cephalalgias (TACs) ^[1]. To tell them apart, your medical team looks at two things: the “stopwatch” (how long the pain lasts) and the “Indo-test” (how the pain reacts to a specific medication) ^[2][3].

The “Stopwatch” Test: Differentiating TACs

The most important clue in diagnosing Paroxysmal Hemicrania (PH) is the timing and frequency of the attacks. PH lives in a “middle ground” between other types of headaches:

• PH vs. Cluster Headache: Cluster headaches are often called the “big brother” of PH. However, Cluster Headache attacks are much longer—usually lasting between 15 and 180 minutes—and happen less frequently (up to 8 times a day) ^[2][4]. PH attacks are shorter (2 to 30 minutes) and happen much more often ^[1].
• PH vs. SUNCT/SUNA: These conditions involve incredibly brief “stabs” of pain that last only seconds to a few minutes ^[5][6]. They can happen hundreds of times a day and are often triggered by touching the face or washing your hair, which is less common in PH ^[5].
• PH vs. Hemicrania Continua: While both respond to the same medication, Hemicrania Continua (HC) involves a continuous, 24/7 background ache that never goes away, even when the sharp “spikes” of pain aren’t happening ^[7][8]. In PH, there is typically a period of total, 100% relief between attacks ^[1].

The Indomethacin Test (The “Indo-Test”)

The “gold standard” for confirming a PH diagnosis is a diagnostic trial of a medication called indomethacin ^[1][3]. It is a potent NSAID (non-steroidal anti-inflammatory drug), and while its precise mechanism in PH remains uncertain, its distinct clinical response is diagnostically useful ^[9].

How the Test Works

Your doctor will likely prescribe a specific “trial” dose. For example, some specialists might start an adult trial at 25 mg taken three times a day ^[10]. If there is no response, the dose may be increased gradually—sometimes up to 75 mg or even 150 mg per day to ensure the test is thorough ^[11][12]. These are strictly clinician-directed examples; never start, increase, or repeat a trial without a prescriber.

What a “Complete Response” Means

In the world of PH, “complete” means absolute ^[1].

• A Positive Result: The headache attacks vanish entirely, sometimes within 24 to 48 hours of reaching the correct dose, while others respond later during a supervised titration ^[10][13].
• A Negative Result: If the pain does not stop completely at a therapeutic dose, the diagnosis of PH is usually reconsidered, though a negative result can also reflect inadequate dose, poor absorption, or intolerance ^[1].

Why a “Partial Response” Matters

It is a common misconception that if a headache “gets better” with indomethacin, it must be PH. However, many types of headaches (including some migraines) might feel a little better after taking a strong anti-inflammatory ^[12].

For a formal diagnosis of Paroxysmal Hemicrania, the response must be absolute ^[1][14]. If your attacks become less frequent or less intense, but do not stop entirely, your doctor will likely reconsider your diagnosis and may investigate other causes, such as Cluster Headache or a “secondary” headache caused by another underlying issue ^[1][15]. Because high-dose indomethacin can be hard on the stomach and kidneys, doctors want to be certain of the diagnosis before committing to long-term treatment ^[16][17].